Conventionally, a combustion apparatus, such as an internal combustion engine, uses a spark plug for igniting an air-fuel mixture through spark discharge. In recent years, in order to meet demand for high output and low fuel consumption of the combustion apparatus, a plasma jet ignition plug has been proposed, since the plasma jet ignition plug provides quick propagation of combustion and can more reliably ignite even a lean air-fuel mixture having a higher ignition-limit air-fuel ratio.
The plasma jet ignition plug includes a tubular insulator having an axial bore, a center electrode inserted into the axial bore in such a manner that a front end surface thereof is retracted from a front end surface of the insulator, a metallic shell disposed externally of the outer circumference of the insulator, and an annular ground electrode joined to a front end portion of the metallic shell. Also, the plasma jet ignition plug has a space (cavity) defined by the front end surface of the center electrode and a wall surface of the axial bore. The cavity communicates with an ambient atmosphere via a through hole (through hole portion) formed in the ground electrode. Additionally, generally, the ground electrode is provided in such a condition that its surface on a side toward the insulator is in surface contact with the front end surface of the insulator. For example, see Japanese Patent Application Laid-Open (kokai) No. 2007-287666, “Patent Document 1”).